InfraredFebruary 1961 Popular
Electronics

Wax nostalgic about and learn from the history of early electronics. See articles
from Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.

It
was 1958, over the Taiwan Strait, when the first Sidewinder air-to-air
missile flew up the tailpipe of a MIG-17 Fresco after being launched
by an F-104 Starfighter. The age of offensive infrared (IR) warfare
had begun. It is amazing that the detection and guidance system
for the Sidewinder was built largely from discrete components, without
the advantage of large scale integration. IR night vision devices
were actually used by the military as early as WWII with the renowned
Snooperscope (handheld) and the
Sniperscope (mounted to a carbine rifle). Both required an infrared
light source to illuminate the target. Today's night vision goggles
and scopes are sensitive enough to be totally passive. This "Infrared"
article from the February 1961 Popular Electronics talks
about early developments in infrared technology. Corny-looking prototypes
of IR wireless phones are shown, but obviously that concept never
took off. There is a cool photo of a 15" diameter solid germanium
disc lens that has been polished for use as an IR missile detection
system "picture window."

During
the Quemoy crisis two years ago, a new weapon made the headlines.
Known as the "Sidewinder," the instrument was the first guided missile
to destroy enemy aircraft in actual combat. The Chinese Nationalists
exploded scores of Red MIG-17 jets in mid-air with the aid of this
lethal device.

Interestingly enough, the Sidewinder missile
is named after the desert rattlesnake - both strike by homing in
on the infrared radiations which their targets emit. The hot exhaust
of an enemy jet becomes the target for this heat-seeking missile,
which zooms into the tailpipe and destroys the aircraft in a fiery
explosion.

What Is Infrared? Actually an
electromagnetic radiation much like radio or light waves, infrared
is produced to some extent by every object above absolute zero (-237°
C). And the hotter an object becomes, the more infrared is emitted
- the sun, for example, is an excellent infrared radiator.

Image tube (left) converts invisible infrared
into visible light; the World War II "snooperscope." which enabled
troops to see in the dark, employed such a detector. Modern
Single-crystal Infrared detector (right) is a byproduct of transistor
development unit; produces output voltage in proportion to amount
of infrared radiation failing upon it. Both devices are products
of Radio Corporation of America (RCA).

The discovery of infrared took place over 160 years ago. In the
year 1800, William Herschel placed a number of thermometers along
the full length of a rainbow-like spectrum of the sun's light which
had been dispersed by a glass prism. As he expected, the thermometers
were heated by this visible light - from the violet at one end of
the spectrum to the red at the other. But there was one thing Herschel
didn't expect. The thermometers were also heated at the end of the
spectrum beyond the visible red region, indicating that some form
of energy was also present there! Since this radiation was below
the visible red region, he dubbed it "infrared."

For
over a century, Hershel's discovery remained nothing more than a
scientific curiosity. Then, in the 1920's and 1930's, several laboratory
instruments were developed which used infrared to identify unknown
materials and analyze chemical compounds. And World War II brought
the amazing "snooperscope" which enabled our troops to literally
see in the dark.

Types of Systems.
Infrared systems are classified into two groups. In the "active"
system, the target is illuminated by an infrared spotlight; the
snooperscope is an example of a device which uses this system. The
second or "passive" system detects the infrared energy emitted by
the target itself, as does the Sidewinder missile. As might be expected,
the passive system requires a very sensitive detector, since the
amount of infrared which objects emit is often extremely small.

The basic instrument used for measuring infrared radiation
is the radiometer. Acting somewhat like the more common photocell,
the radiometer collects radiation from a narrow field and converts
it into electrical energy which can then be read on a meter or recorded
on a chart. Radiometers are used to monitor temperatures remotely
with a very high degree of accuracy.

Applications.
Infrared has been applied to a wide variety of guided missiles,
with air-to-air types-such as the Sidewinder - the most successful.
Another military application is in aircraft gunfire control. Fire-control
systems using visual sights are naturally limited to daytime operation,
but infrared equipment extends the operation of such systems into
total darkness.

Still
other military applications include airborne early-warning systems,
ballistic-missile detection systems, "passive" viewing systems for
watching troop movements at night, and infrared communications systems.
The "Midas" satellites, for example, will have infrared "eyes" to
detect the white-hot exhaust of missiles as they are launched from
enemy territory.

Many commercial applications of infrared
stem from one fact: as infrared light is passed through a chemical
compound, certain wavelengths are absorbed and do not pass through.
These wavelengths or groups of wavelengths are known as absorption
bands. And because the molecules of every substance have a different
infrared absorption band, these bands provide a means of identifying
molecules in much the same way that a set of fingerprints can identify
a particular human being.

The infrared spectrophotometer,
for example, is an instrument which analyzes compounds and gases.
It automatically measures the changes in wavelength of the infrared
light passing through a sample and records the resultant absorption
bands on a chart.

Infrared finds dozens of uses in industry.
Among them: analyzing fertilizers, insecticides, and soils in agriculture;
complex propellant mixtures and exhaust gases in aircraft and missiles;
molecular structure of enzymes and amino acids in biochemistry;
essential oils and mixtures in cosmetics; compounds in pharmaceutics.

Fire detection is also becoming an important application
for infrared. Airlines are now using infrared fire-detection devices
aboard their planes, and railroads call on infrared to detect "hot"
boxes from fixed positions - even though the trains are moving by
at high speeds! Some day, a greater degree of protection from forest
fires may be provided through the use of small, battery-operated
infrared devices.

Wireless telephone - the
"Infraphone" - sends conversations hundreds of feet over invisible
infrared beam. Produced by Infrared Industries, it is fully transistorized
and operates on flashlight batteries. It' available from most major
radio supply houses.

Place in the Spectrum.
All of the advanced military systems now in use, as well as many
of the commercial applications, were made possible by new developments
in detectors and optics since World War II. Before taking a closer
look at infrared detectors, let's see just how infrared fits into
the whole of the electromagnetic spectrum.

The infrared
frequency band, which is located between visible light and radar,
ranges from approximately 1 million to 500 million megacycles; the
corresponding wavelengths are 1000 to 0.75 microns. (The micron
- actually one millionth of a meter - is the unit commonly used
for measuring wavelengths in infrared work).

In some of
its characteristics, infrared resembles visible light - for example,
lenses and parabolic mirrors are used to collect and focus infrared
energy on a detector. However, it also behaves somewhat like radio
or radar waves: it will go right through materials such as germanium
and silicon, both of which are impervious to visible light!

Traffic detector - the "Traf­fitrol" - uses
infrared beam to detect and count vehicles traveling at speeds
up to 80 miles per hour. Highly accurate and virtually foolproof,
the device is among the first to apply infrared principles to
traffic problems. The "Traffitrol" is manufactured by the Heiland
Division of Minneapolis-Honeywell.

Infrared "picture window" produced by scientists
at Hughes Aircraft is solid germanium casting 15" in diameter
and 1/2" thick. Optically ground and polished, it is opaque
to ordinary light but refracts infrared rays much as a glass
lens collects and focuses rays from visible part of spectrum.
Used in missile detection systems, the casting is made from
9 1/2 pounds of germanium, and is valued at approximately six
thousand dollars.

Infrared Detectors. One type of infrared detector,
the image tube, operates only in the near-infrared region. The object
to be viewed is irradiated with an infrared spotlight, a device
much like an ordinary spotlight, except that it makes use of a filter
which lets only the infrared pass. The infrared is reflected from
the object and strikes a sensitive film in the image tube.

Since this film is photoemissive, it emits electrons when excited
by the infrared light. The electrons are emitted from the back side
of the film into the vacuum within the image tube and electrostatically
focused on a phosphor viewing screen. Thus, the image tube effectively
converts the invisible infrared to visible light and enables the
viewer to "see" in total darkness.

Infrared detectors, other
than the photoemissive type, fall into one of three groups - thermal
radiation detectors, film-type infrared photoconductors, and single-crystal
infrared detectors. Since they are detectors, all convert infrared
radiations into electrical signals. But each works differently and
therefore has special characteristics all its own.

The thermal
radiation detectors make use of the heating effects of infrared.
There are two types: one is the thermoelectric detector which operates
on the thermocouple principle. It consists of two dissimilar metals
which generate voltage at the junction as the temperature of the
junction changes. Naturally, the junction temperature is proportional
to the amount of radiation hitting it.

The second type of
thermal detector is the bolometer. It consists of a thin metal or
semiconductor strip. As the temperature of the strip changes, so
does the resistance. And if voltage is applied across the bolometric
strip, the current flowing through it will vary as its resistance
changes.

Photoconductive detectors are comprised of a thin
film (about 1 micron thick) deposited on a thin sheet of insulating
material such as glass. For their operation, they rely on the photoconductive
effects of certain semiconductor compounds. The radiation changes
the conductivity of the material in much the same manner that base
bias controls current flow in a transistor.

This effect
can be demonstrated by connecting a bias battery and a sensitive
current meter in series with the detector. As radiation falls upon
the detector, current will flow. And by replacing the meter with
a load resistor, a signal will be developed across it in proportion
to the amount of radiation.

Single-crystal infrared detectors
were made possible largely by the invention and development of the
transistor. In this type of detector, a semiconductor crystal is
used, generally comprised of either germanium or silicon. The material
is processed in such a way as to make it photovoltaic, which means
that it generates a small d.c. voltage proportional to the amount
of radiation falling on it.

In all of these detectors, the
signal voltages produced are very small. As a result, special low-noise
amplifiers are required to bring the signal up to more usable levels.

Tomorrow's Promise. As knowledge of infrared
and its amazing properties increases, so, too, will its uses. A
new traffic-control detector, for example, can detect and count
cars traveling at speeds up to 80 miles an hour. Another recent
development - a solid casting of germanium, 15 inches in diameter
- expands the viewing range of infrared missile detection systems
enormously (scientists have termed it comparable to "replacing a
porthole with a picture window!") Even infrared detectors have reached
all but unbelievable levels of sensitivity. One recent model, already
on the market, is so sensitive that it can detect a cigarette burning
500 miles away!

Fantastic? To be sure, but this is the fantastic,
invisible world of infrared. What still more wonderful developments
in this invisible world will tomorrow bring?

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